1. Field of the Invention
The present invention relates to apparatuses for reducing power leaking from a transmitter side to a receiver side, for use in communication equipment which performs communication by allocating two frequencies in adjacent frequency bands to transmission and reception.
2. Description of the Related Art
In communication equipment which performs communication by allocating two frequencies in adjacent frequency bands to transmission (transmission frequency f1) and reception (reception frequency f2), a duplexer is provided to use an antenna for both transmission and reception, as shown in FIG. 14.
The structure shown in FIG. 14 will be described. A transmitter 10 includes a power amplifier (PA) and outputs a transmission signal. A receiver 20 includes a low-noise amplifier (LNA) and detects a desired signal. An antenna 30 emits the transmission signal in the form of a radio wave into space and also catches a desired reception signal out of radio waves coming from space. A circulator 110 is inserted to protect the power amplifier PA from damage caused by part of the transmission power being reflected back to the output side of the transmitter by the antenna 30 or the like. The circulator has three ports having nonreciprocal properties: The power input from a first port P1 is transmitted to a second port P2, ideally without loss, and the power input from the second port P2 is transmitted not to the first port P1, but to a third port P3, ideally without loss.
A duplexer 120 has three terminals, and its internal structure is as shown in FIG. 15. The duplexer 120 includes a transmission band-pass filter 121 (BPF 1) which allows a signal in the transmission frequency band to pass through it with a small loss and which sufficiently suppresses a signal in the reception frequency band, and a reception band-pass filter 122 (BPF 2) which allows a signal in the reception frequency band to pass through it with a small loss and which sufficiently suppresses a signal in the transmission frequency band. The signal in the transmission frequency band can go between a first terminal #1 and a second terminal #2, but the signal in the reception frequency band is suppressed there. The signal in the reception frequency band can go between the second terminal #2 and a third terminal #3, but the signal in the transmission frequency band is suppressed there. The signals in the transmission frequency band and the reception frequency band are both suppressed between the first terminal #1 and the third terminal #3. The transmitter 10 is connected to the first terminal #1 through the circulator 110, the antenna 30 is connected to the second terminal #2, and the receiver 20 is connected to the third terminal #3. Ideally, the transmission signal in the transmission frequency band sent from the transmitter 10 is input to the first terminal #1 and output through the second terminal #2 to the antenna 30; and the reception signal in the reception frequency band received by the antenna 30 is input to the second terminal #2 and output through the third terminal #3 to the receiver 20. A terminator 710 is usually a 50-ohm resistor which absorbs the signal output to the third port P3 of the circulator 110 (consumes the current by converting it to heat).
With that structure, the transmission signal is sent to the antenna 30 with a small loss and is emitted in the form of radio waves into space. Part of the power of the transmission signal is reflected back to the duplexer 120 because of the reflection characteristics of the antenna 30 and passes through the BPF 1 with a small loss. The reflected signal reaches the circulator 110, and then it is transmitted to the third port P3 and absorbed by the terminator 710. Therefore, the reflected signal will not adversely affect the PA in the transmitter 10.
The power of the transmission signal reflected by the antenna 30 is transmitted also to the BPF 2, and most of the power is suppressed by the BPF 2. However, a very small part of the reflected power leaks to the receiver side. If the power of the signal input to the antenna 30 is large and if the absolute value of the reflected power is large accordingly, the leakage power to the receiver side may cause distortion in the LNA.
The same problem occurs between the terminals #1 and #3, where the duplexer 120 is connected to the transmitter and the receiver, because a part of the power of the transmission signal leaks to the receiver side.
The simplest way to solve the problem caused by the power of the transmission signal leaking to the receiver side would be to improve the out-of-band suppression characteristics of the BPF 1 and BPF 2 included in the duplexer 120 or to lower the degree of coupling between the terminals. However, it is very difficult to adopt this approach if the transmission frequency is close to the reception frequency.
A different solution disclosed in Japanese Patent Application Laid Open No. H2-151130 uses a leakage power reduction apparatus 800 structured as shown in FIG. 16. The leakage power reduction apparatus 800 includes a divider 810, a circulator 820, a level adjuster 830, a phase shifter 840, and a combiner 140. The divider 810 divides the power of the transmission signal input from the transmitter 10 into two parts. The circulator 820 has nonreciprocal properties: The power input from a first port P1 is transmitted to a second port P2, ideally without loss; and the power input from the second port is transmitted not to the first port, but to a third port P3, ideally without loss. The divider 810 is connected to the first port, the antenna 30 is connected to the second port, and the combiner 140 is connected to the third port. The level adjuster 830 adjusts the amplitude of the signal input from the divider 810 and outputs the adjusted signal to the phase shifter 840. The phase shifter 840 adjusts the phase of the signal input from the level adjuster 830 and gives the adjusted signal to the combiner 140. The combiner 140 is connected to the third port of the circulator 820 and to the phase shifter 840 and combines the signals input from them and gives a combined signal to the receiver 20.
The principle of operation of the apparatus will be described next. The transmission signal coming from the transmitter 10 passes through the divider 810 and the circulator 820 and is emitted from the antenna 30. Part of the power of the transmission signal would be reflected by the antenna 30 and leak to the receiver side through the third port of the circulator 820. In this apparatus, however, part of the power of the transmission signal from the transmitter 10 is extracted from the divider 810. The amplitude and phase of the extracted signal are adjusted to generate an offset signal having the same amplitude and the opposite phase with respect to the leakage signal when combined in the combiner 140. The leakage power is suppressed by combining the offset signal and the leakage signal in the combiner 140.
A different solution disclosed in the abstract of Japanese Patent Application Laid Open No. H9-116459 uses a leakage power reduction apparatus 900 structured as shown in FIG. 17. The leakage power reduction apparatus 900 includes circulators 110, 820, an amplifier 910, a phase shifter 840, and a combiner 140. The circulator 110 is the same as the one included in the structure described earlier with reference to FIG. 14. The circulator 820, the phase shifter 840, and the combiner 140 are the same as the corresponding elements included in the apparatus described earlier with reference to FIG. 16. Elements having the same functions as those shown in FIG. 14 or FIG. 16 will be given the same reference numbers in FIG. 17, and a description thereof will be omitted. Like numbering will also be used in the other drawings. The amplifier 910 amplifies a signal input from a third port of the circulator 110 and gives the amplified signal to the phase shifter 840.
The principle of operation of the apparatus will be described next. A transmission signal coming from the transmitter 10 passes through the circulator 110 and the circulator 820 and is emitted from the antenna 30. Part of the power of the transmission signal would be reflected by the antenna 30 and leak to the receiver side through a third port of the circulator 820. This apparatus, however, uses the phenomenon that a portion of the part of the power of the transmission signal reflected by the antenna 30 leaks through a second port to a first port of the circulator 820. The corresponding leakage power is extracted from the third port of the circulator 110, and the extracted signal is used to generate an offset signal. Since the power of the extracted signal is weak because of the isolation function of the circulator 820, the amplifier 910 amplifies the signal. Then, the phase is adjusted to generate an offset signal having the same amplitude and the opposite phase with respect to the leakage signal when combined in the combiner 140. The leakage power is suppressed by combining the offset signal and the leakage signal in the combiner 140.
In the solution disclosed in Japanese Patent Application Laid Open No. H2-151130, since the divider 810 extracts a part of the transmission power to generate the offset signal, the power utilization efficiency of the apparatus is lowered. The solution disclosed in the abstract of Japanese Patent Application Laid Open No. H9-116459 does not waste the transmission power, but because the weak leakage power from the circulator 110 is used, an amplifier is needed. The power consumed by the amplifier inevitably lowers the power utilization efficiency of the apparatus.